Fuel injectors are well known for supplying metered amounts of fuel to combustors such as internal combustion engines, and reformers such as hydrogen/reformate generators for fuel cells. In either case, it is highly desirable that the fuel spray created by these injectors be well atomized for essentially instantaneous vaporization upon entering the spray chamber, whether it be the injection port or firing chamber of an engine or the vaporizer chamber of a catalytic reformer. In a fuel cell, for example, this is a desirable since the liquid fuel is thereby inhibited from contacting the hot metal surfaces of the vaporizer chamber, thus preventing undesirable carbon formation and uncontrolled combustion.
Conventional port fuel injectors operate at lift pump pressures of less than 400 kPa and employ director-style spray tips. A conventional fuel director can have one to ten or more holes that define a spray pattern and flow rate of the injector. As the size and/or number of holes in the director is increased, the flow rate of the injector at a given pressure also increases. The diameter of the hole also determines the spray droplet size. As the hole diameter decreases, the droplet size also decreases desirably at a given pressure; however, if the hole diameter is too small, the holes are susceptible to plugging from fuel and combustion deposits. Therefore, the minimum practical lower limit for a director hole diameter is approximately 100 microns (0.1 mm). This hole size limits the minimum spray droplet size at a 400 kPa lift pump pressure to dv90's of approximately the diameter of the hole; and in practice most droplets are larger. Therefore, a physical barrier (hole diameter) limits the minimum droplet size obtainable with a director style injector spray tip. In addition, the director style spray tip generates sprays that are non-uniform and stringy in comparison to sprays generated by apparatus in accordance with the invention as detailed hereinbelow.
Pressure-swirl atomizers, capable of generating sprays in continuous systems such as paint sprayers and gas turbine nozzles, are well known. Pressure-swirl atomizers have also been applied to pulsed-spray applications, such as fuel cells and high-pressure gasoline fuel injectors, to provide finely atomized sprays.
A pressure-swirl atomizer has several advantages over director-plate atomizers traditionally used for pulsed spray applications. First, pressure-swirl atomizers can produce smaller droplets. This is especially evident at lower pressures, as required by port fuel injection systems. Also, pressure-swirl atomizers are less susceptible to plugging than director type atomizers. Additionally, pressure-swirl atomizers can generate uniform hollow-cone sprays that are most desirable in a direct cylinder injection application.
A disadvantage of prior art pressure-swirl atomizers is that large droplets of fuel, known in the art as a “SAC” spray, are released into the spray chamber at the beginning of each injection pulse. When the injector first opens, the fuel located between the swirler and the valve seat does not have rotational velocity. This fuel exits the injector axially in mostly non-atomized large droplets, not in a finely atomized cone. These large droplets in the SAC spray are undesirable because the fuel contained therein is generally non-metered and can also reach chamber surfaces where it can produce carbon formation in fuel cells, as well as higher emissions from internal combustion engines. Therefore, it is desirable to use an optimized swirler/nozzle design to produce very small droplets in a conical spray pattern as the fuel exits the injector.
Conventional pressure-swirl atomizers typically include a complex swirler constructed of powdered metal. Manufacturing costs associated with the use of powdered metal swirlers are relatively high. Other types of pressure-swirl atomizers utilize flat-plate swirlers stamped from sheet metal. This process typically limits their geometry to simple circular and straight-line passages to keep the stamping tool simple and durable. However, such limitations restrict the performance of the part. Additionally, this process can also result in sharp edges and abrupt transitions that can induce the flow to separate undesirably from the edges, resulting in cavitation erosion of the swirler and unpredictable flow patterns. Such flow separation is quite sensitive to edge conditions such as sharpness or burrs. Slight variations in edges can translate into non-uniformity in the produced parts and resulting flow variations.
What is needed is a pressure-swirl plate for a fuel injector that reduces the cost, flow variation, and transient spray development problems associated with prior art swirl plates, while maintaining their advantages over director-style atomizers.
It is a principal object of the present invention to optimize flat swirler plate geometry to optimize performance of a pressure-swirl atomizer.
It is a further object of the invention to simplify the construction and reduce the cost of producing a swirler-plate nozzle atomizer.